1. Field
The present disclosure generally relates to X-ray Computed Tomography (CT) imaging. In particular, embodiments herein relate to an apparatus, detector, and associated methods for switching ON or OFF a bias voltage of energy-discriminating detector elements based on contour estimation information.
2. Background
Radiographic imaging, in its simplest expression, is an X-ray beam traversing an object and a detector relating the overall attenuation per ray. The attenuation is derived from a comparison of the same ray with and without the presence of the object. From this conceptual definition, several steps are required to properly construct an image. For instance, the finite size of the X-ray generator, the nature and shape of the filter blocking the very low energy X-ray from the generator, the details of the geometry and characteristics of the detector, and the capacity of the acquisition system are all elements that affect how the actual reconstruction is performed.
In one of many possible geometries, the X-ray source on top of the graph shown in FIG. 1 is emitting an X-ray beam forming a fan, traversing the object. While a wide range of values can exist, typically, the distance “C” is around 100 cm, “B” is around 60 cm, and “A” is around 40 cm. The principle of tomography requires that each point of the object is traversed by a collection of rays covering at least 180 degrees. Thus, the entire X-ray generator and detector assembly will rotate around an object. Mathematical considerations show that the tomographic conditions are met when a scan of 180 degrees plus the fan angle is performed.
Conventional X-ray detectors integrate the total electrical current produced in a radiation sensor, and disregard the amplitude information from individual photon detection events. Since the charge amplitude from each event is proportional to the photon's detected energy, this acquisition provides no information about the energy of individual photons, and is thus unable to capture the energy dependence of the attenuation coefficient in the object.
On the other hand, semiconductor X-ray detectors that are capable of single photon counting and individual pulse height analysis may be used. These X-ray detectors are made possible by the availability of fast semiconductor radiation sensors materials with room temperature operation and good energy resolution, combined with application-specific integrated circuits (ASICs) suitable for multi-pixel parallel readout and fast counting.
When operating such a photon-counting X-ray detector, a high bias voltage is applied across the sensor crystal such that the electron-hole pairs generated from the radiation interaction are rapidly swept toward the collecting electrodes. Each radiation interaction event results in a pulse sent to the readout electronics, which undergoes pulse height analysis and is counted.
One major advantage of such photon-counting detectors is that, when combined with pulse height analysis readout, spectral information can be obtained about the attenuation coefficient in the object. Conventional CT measures the attenuation at one average energy only, while in reality, the attenuation coefficient strongly depends on the photon energy. In contrast, with pulse height analysis, a system is able to categorize the incident X-ray photons into several energy bins based on their detected energy. This spectral information can effectively improve material discrimination and target contrast, all of which can be traded for a dose reduction to, for example, a patient.
One challenge with using such photon-counting detectors for medical CT applications is the very high X-ray flux required in most CT tasks. Unlike conventional X-ray detectors, single photon counting requires very fast sensor-crystal response and readout-electronics response. In a routine CT scan, as many as 108 photons, or even more, can hit one detector element every second. At such high flux, the electron and hole carriers generated in the sensor do not have enough time to be fully collected at the electrodes and removed from the crystal bulk. For many semiconductors of interest for X-ray detection, this is especially pronounced for hole carriers, which travel more than ten times slower than the electrons. This, combined with crystal imperfections and defects that further trap the carriers, results in building up of charges from the uncollected carriers and an internal electric field.
This internal electric field (“Eint”) is of opposite direction to the external electric field generated by the applied bias (“Tbias”). As a result, the net electric field inside the crystal is weakened, further preventing the full collection of the charge carriers. The net result will be the inability of the detector to respond to incoming radiation and count loss in the measured data. This phenomenon, referred to as “polarization,” prevents semiconductor photon-counting detectors from fully realizing their potential in high-flux CT applications.